Optical scanning system with a crossed scanning pattern

ABSTRACT

An optical scanning system for horizon scanners, and the like, which provides a dual-lobe crossed scanning pattern. A detector arrangement receiving radiation from the scanner provides sufficient signals for determining the attitude of a space vehicle with reference to a planet about which the space vehicle is orbiting.

United States Patent [1 Harper 1111 3,793,518 [4 1 Feb. 19,1974

1 1 OPTICAL SCANNING SYSTEM WITH A CROSSED SCANNING PATTERN [75]Inventor: Kennard W. Harper, Endwell, NY.

[73] Assignee: Ithaco Inc., Ithaca, N.Y.

[22] Filed: Mar. 17, 1972 21 Appl. No.: 235,687

[52] US. Cl. 250/83.3 H, 250/83 R [51] Int. Cl. G0lj U112 [58] Field ofSearch 250/83 R, 83.3 H

[56] References Cited UNITED STATES -PATENTS 3,083,611 4/1963 Ziolkowskiet a1. 250/833 H X 8/1965 Brumfield et a1.

3,204,101 250/833 H 3,612,879 10/1971 Ohman 250/83.3 H 3,631,248 12/1971Johnson 250/833 H Primary Examiner-Archie R. Borchelt Attorney, Agent,or Firm-Charles C. Krawczyk [5 7] ABSTRACT An optical scanning systemfor horizon scanners, and

the like, which provides a dual-lobe crossed scanning pattern. Adetector arrangement receiving radiation from the scanner providessufficient signals for determining the attitude of a space vehicle withreference to a planet about which the space vehicle is orbiting.

28 Claims, 15 Drawing Figures PATENTEDFEBIQ 1914 3393518 sum 1 or 9SECTION AA PATENTEDH-IB 1 9 m4 3793518 SHEEI 5 BF 9 LENS (JENTER LINE 50SCAN PATH OPTICAL SCANNING SYSTEM WITH A CROSSED SCANNING PATTERNBACKGROUND OF THE lNVENTlON This invention pertains to optical scanningsystems in general, and more particularly to an optical scanner forproviding a dual-lobe crossed type of scanning pattern for use withhorizon sensors, and the like.

When high flying aircraft, such as satellites and space craft, orbit aplanet such as the earth, at a distance wherein the planets gravity cannot be effectively used to reference the orientation of the aircraft,other means, such as horizon sensors are required to provide attitudereference information. The reference information is used to maintain thevehicle at a constant attitude relative to the planet and thereforereduce any wobble or attitude error to an acceptable level. The horizonsensors provide information, preferably in the form of electricalsignals, that correspond to the departure of the vehicle from apredetermined attitude. Attitude correction control systems, such asthose employing torque wheels, gas jets, plasma jets, and the like,respond to the electrical signals to maintain the vehicle at a constantattitude. Such attitude correction control systems should be adaptableto both high and low orbits, synchronous orbits, and also highlyelliptical orbits, without optical or mechanical modifications.

The attitude of a satellite is determined by its position with respectto three axes of rotation (or three angular degrees of freedom) locatedat right angles to each other. Two of the axes (pitch and roll) lie in aplane normal to a projected radius of the earth passing through thesatellite and the third axis (yaw) coincides with such radius. The rollaxis (in the direction of vehicle motion) and the pitch axis (in adirection normal to vehicle motion) lie ,in a plane parallel to theplanets horizon. The information pertaining to the roll and pitch axescan be used to control yaw. For example, with an attitude control systemthat applies correction torques to wheels located in the pitch-yarplane, yaw control can be accomplished by orbital coupling, wherein, ayaw error existing at one point of the orbit becomes a roll error aquarter of an orbit later and thus eventually all errors are sensed andcorrected. Other control systems can use gyroscopes, or observation ofheavenly bodies, to provide yaw orientation.

The horizon of a planet represents a line of discontinuity between theplanet s atmosphere and outer space. The line of demarcation between theearth and outer space i.e., the earths horizon, provides a markeddifference in infrared radiation. Outerspace is very cold and providesvery little infrared radiation, while the earth s atmosphere is muchwarmer and provides a substantially greater amount of infraredradiation. Although the scanning system of the present invention will bedescribed herein as an infrared earth horizon scanner (since this is themost important and practical field of utility at the present), it is tobe understood,

that other types of radiation can also be employed, de-

pending upon the planet to be orbited. Therefore, reflected visiblelight, or ultraviolet radiation, or any other radiation emitted or'reflected by the planet, can also be used depending upon conditionssurrounding the planet being orbited.

The horizon scanners presently inuseinclude apairof infrared scanners,each providing a conical type scan, wherein each of the scanners areoriented on opposite sides of the vehicle having the axes of the conicalscan lying in the roll-yaw plane. The electrical signals from thescanners corresponding to the crossing of the earths horizon, aretranslated into pitch and roll error signals for attitude correction. Inaddition, the electrical signals can be processed to provide anindication of altitude. A scanning arrangement of this type is disclosedin a U.S. Pat. No. 3,020,407, issued to M.M. Merlen and entitled HorizonSensor. Although horizon sensors of this type have been successfullyused in the past, such horizon sensors require duplication of partsresulting in added weight, power consumption and multiplication ofmoving parts, thereby effecting the overall statistical life expectancyof the vehicle.

A single scanning arrangement has been developed in the prior art thatincludes two prisms rotated in opposite directions at a predeterminedspeed ratio. A scanning arrangement of this type is disclosed in a U.S.Pat. No. 3,083,61 l, issued to Adrian J. Ziolkowski et al and entitledMulti-Lobe Scan Horizon Sensor. The counter-rotating prisms produce amulti-lobe crossed scanning pattern having at least three lobes. Withthis arrangement, the number of lobes to be used in the scanning patternis determined by the speed of the scan relative to the response of thedetection devices, the desirability for an even number of lobes, thewidth of the lobes, etc. In the particular embodiment disclosed, a fourlobe scanning pattern was preferred.

Although, this multi-lobe scanning arrangement did eliminate some of theproblems of duplication of parts, the arrangement still requires twomoving optical elements, large bearings, and corresponding geardriveunits. In addition, the arrangement also requires, preferably, at leasta four lobe scan resulting in the detection of eight horizon crossingsfurther resulting in added complications in signal processing.Furthermore, if it is desired to reduce the width of the lobes of thescan pattern, the relative difference between the speeds at which theprisms are rotated is increased, possibly resulting in a conditionwherein the speed of the scan will be too fast for the detectiondevices, or else possibly creating undesirably long time constants inthe control system if the scan rates are reduced to stay within theresponse time of the detection devices.

The optical scanners of this type require two sets of moving parts, onefor each prism. Due to the low'pressure experienced by vehiclesorbitingin outer space, it is desirable to maintain such moving'parts inpressurized units to minimize lubrication problems. In any event, itishighly desirable to minimize the number of moving parts, their mass, andthe rate at which the a new and improved optical scanningsystemproviding a duallobe crossed scan pattern wherein the width of thelobe can be changedwit h'out increasing-the speed of the scan, orrequiring additional lobes in the scanning pattern.

It is another object of this invention to provide a new and improvedoptical scanning system for horizon sensors that reduces the weight andpower requirements and reduces the number of moving parts therebyincreasing the systems reliability.

It is still a further object of this invention to provide a new andimproved single scanning system for horizon sensors that is adaptible tohigh and low orbits, synchronous orbits, and elliptical orbits withoutmodification.

It is another object of this invention to provide an optical scanningarrangement for transmitting a beam of radiation in a dual-lobe crossedscanning pattern that only requires a single moving optical element.

BRIEF DESCRIPTION OF THE INVENTION The scanning system of the inventionprovides a duallobe crossed type of scanning pattern. When used withhorizon sensors, radiation is received by the scanning system from theplanet which is being orbited and is directed toward detection means forproviding electrical signals corresponding to the attitude of the spacevehicle relative to the planets horizon. In an alternative embodiment,radiation from a source, such as a laser, is directed at the scannersystem, wherein the scanning system projects a radiation beam along thedual-lobe crossed scanning pattern.

In accordance with the invention the scanner includes at least oneoptical element, such as a prism,

having a relfective surface, mounted for rotation on an axis which is atan angle transverse to the reflecting surface and wherein the centerline of the optics of the detector, or radiation source, is located atan angle transverse to the axis of rotation, and usually normal thereto.In the preferred embodiment, a double dove prism is used having tworight angle prisms positioned with their reflecting surfaces (hypotenae)abutting each other.

A further feature of the invention includes the use of a three flakedetector for providing an automatic sun radiation rejection function.One of the three flakes functions as a common flake. The common flakeisused with one of the other flakes during one half the dual-lobe crossedscanning pattern, and is used with the other flake during the other halfof the scanning pattern.

Control circuit means is provided, responsive to signals generated bythe detector means, for producing control signals for correcting theattitude of the space vehicle.

BRIEF DESCRIPTION. OF THE FIGURES FIG. 1 illustrates a verticalsectional view through a horizon sensor including the optical scanningsystem of the invention.

FIG. 2 illustrates a top sectional view of the optical scanner portionof the horizon sensor of FIG. 1 taken along the lines A*A.

FIG. 3 illustrates an isometric view of the prism of FIGS. 1 and 2 withthe axis of rotation intersecting the prism reflective surfaces.

FIGS. 4A and 4B illustrate an optical schematic diagram of the scanningsystem of the invention, as viewed along the axis of rotation, and asviewed normal to the axis of rotation, respectively, for a zero degreescan position.

FIGS. 5A and 58 illustrates an optical schematic diagram of the scanningsystem of the invention, as viewed along the axis of rotation, and asviewed normal to the axis of rotation, respectively, for a scan positionin the order of 45 degrees.

FIG. 6 illustrates the dual-lobe crossed scanning pattern of the opticalscanning system of the invention in a position corresponding to correctvehicle attitude.

FIG. 7 illustrates a three flake detector arrangement for use with theoptical scanning system of the invention, for sun radiation rejection.

FIG. 8 illustrates the position of the optical scanner of the inventionrelative to the roll and pitch axis of the vehicle and in a positioncorresponding to a correct attitude position relative to the earth,designating the directions of the scan paths and the sequence at whichthe three flake detectors of FIG. 6 receive radiation signals from thescanner depending upon the particular path scan in the scan pattern. a

FIG. 9 illustrates a schematic diagram of a control circuit responsiveto signals generated by the three flake detector arrangement of FIG. 7for producing control signals for controlling the attitude of the spacevehicle.

FIG. 10 illustrates a series of waveforms disclosing the signalsprocessed by the control circuit of FIG. 9 with a zero attitude error.

.FIG. 11 illustrates a plot of the electrical waveforms of the controlcircuit of FIG. 9 wherein a pitch error is detected.

FIG. 12 illustrates a plot of the electrical waveforms of the controlcircuit of FIG. 9 wherein a roll error is detected.

FIG. 13 illustrates a optical schematic diagram of laser beam scanneraccording to a second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The horizon sensor of FIG. 1includes a motor drive unit 10 and an optical scanning unit 12. Thedrive unit 10 includes a reaction wheel arrangement including a variablespeed momentum wheel powered by a two phase induction motor (not shown).The motor is of a constant torque design, that is, the available torqueis substantially independant of operating speed. The rotor 14 is outsidethe stator (not shown) to maximize momentum. The angular position of therotor 14 is detected by (four) magnetic detectors l6A-l6D mounted on thebase 11 andlocated at degree intervals adjacent to the circumference ofthe reaction wheel rotor 14. A permanent magnet 18 is fastened to rotor14, which in turn induces a reference signal in the magnetic detectors16A-l6D as the magnet passes the individual detectors.

The optical scanning unit 12 includes, in the preferred embodiment, adouble dove prism 20 suitably fastened to a shaft 22 by a strapmechanism 21 so that the prism 20 is rotated by the motor driven unit10. The double dove prism 20 consists of two right angle Harting-Dovetype prisms 24 and 26, each having a silvered reflecting surface 28. Thetwo prisms 24 and 26 are cemented together with their reflectingsurfaces 28 (hypotenae) abutting and are secured to the shaft 22 by thestrap mechanism 21, wherein the double dove prism takes the shape of arectangular shaped cube. Two opposite corners 23 and 25 of the cube arecut away or beveled to simplifythe securing thereof by the strapmechanism 21. As will be shown in a later portion of the specification,only a portion of the scanning pattern of the double dove prism 20 isemployed for horizon sensing and therefore the strap mechanism 21 willnot interfere with the scanning operation. In the particular embodimentof the invention as illustrated in FIGS. 1 and 2, the double dove prism20 is mounted for rotation about an axis that passes through oppositecorners 27 and 29 of the cube and transverse a plane including thereflecting surfaces 28. In the embodiment illustrated in FIGS. 1, 2 and3, the plane including the reflecting surfaces 28 is displaced relativeto the axis of rotation 30 so that an angle A on the order of 45 degreesis formed between the axis 30 and a line 31 normal to the plane. Hence,as the prism 20 is rotated about the axis 30, the reflecting surfaces 28provides a wobbling motion relative to the axis.

The double dove prism 20 receives parallel or collimated radiation andtransmits the same to a focusing lens 32. The focusing lens 32, in turn,focuses the beam of radiation upon a detector 34, illustrated in thisparticular embodiment as a hyper-immersed hemisphere bolometer. The axis37 of the lens 32 intersects the double dove prism 20 'in the center ofthe plane 28 (.prism hypotenuse) and also intersects, and is normal to,the axis of rotation 30. Hence, as the double dove prism 20 is rotatedabout the axis 30, each of the reflecting surfaces 28 of the prisms 24and 26 face in the general direction of the lens 32 during opposite 180degree portions of the rotation of the shaft 22.

The double dove prism 20, as it is rotated, provides a dual-lobecrossed, or figure eight, type scanning pattern 35 as illustrated inFIG. 6. It is to be understood, that the scanning pattern 35, asillustrated in FIG. 6, shows a general representation of the dual-lobecrossed shape pattern and is not a scale representation. The portion ofthe scanning pattern 35 near the crossover point 36 illustrated by thesolid lines designates a linear or nearly linear postion of the scantrace that can be used for sensing of the earths horizon 37. The portionof the scanning unit 12 (FIG. 1) facing the earth is covered with aradiation transmitting dome 38. Since only the portion of the scanningpattern near the crossover 36 is used for horizon sensing, the dome 38can be covered with a metallic shell 40 having an X type cut-away 42 andthereby provide added supporting strength to the dome 38. The X typecut-away 42 provides a radiation transmitting path for the linearportion of the scanning pattern.

FIGS. 4A and 4B illustrate the position of the double dove prism 20 in azero degree rotational position which corresponds to the crossover point36. The radiation received from the crossover point 36 is transmittedalong the centerline 37 of the lens 32 to the detector 34 (asillustrated by the dashed lines). The rotation of the prism 20 about theaxis 30 produces a scan coverage at twice the shaft rotation. This isillustrated in FIGS. 5A and 58, wherein the prism 20 is rotated to anangle of A degrees relative to the center line 37 to produce a scanangle of 2A degrees. Since the rotation of the double dove prism 20produces twice the scan angle, a single rotation of the double doveprism 20 around the axis 30 produces the dual-lobe crossed scanningpattern 35 of FIG. 6.

It can be assumed that the reference point '1 (detected bymagneticsensor 16A) of the scanning pattern 35 corresponds to the zerodegree position of the double dove prism 20 as illustrated in FIGS. 4Aand 4B. As the double dove prism 20 is rotated in the direction asdesignated, the scanning system scans in the direction of scan pathNo. 1. When the double dove prism 20 is rotated an angle of degress thereference point 2 is reached and detected by the magnetic sensor 168.Further rotation of the prism 20 in the same direction causes the scanpattern to are back towards the'crossover point 36 along scan path No.2. When the double dove prism 20 is rotated an angle of degrees, thecrossover point 36 is reached a second time and the reference point 3 isdetected by the magnetic detector 16C. As the double dove prism 20 isfurther rotated, the scanning pattern continues to follow in the samedirection along the scan path No. 2. When the rotation angle of 270degrees is reached, the reference point 4 is detected by the magneticsensor 16D. Still further r0- tation causes the scanning pattern tocurve back towards the crossover point 36 along the scan path No. 1.When a full 360 degree rotation angle is reached, the reference point 1is detected a second time by the magnetic sensor 16A. Therefore, it canbe seen that a complete rotation of the double dove prism 20 about therotational axis 30 produces the dual-lobe crossed scanning pattern 35having a linear or substantially linear portion designated by the soliddark lines. This nearly linear portion is used for horizon sensing.Since only a portion of the scanning pattern is used, the X shaped cutaway 42 in the metallic shell 40 (FIG. 1) need only be large enough toexpose the selected crossed portion of the scan pattern. In addition,the strapping mechanism 21 does not block the linear portion of thescanning pattern.

With the plane of the reflecting surfaces 28 tilted to define an angle Abetween the line 31 normal to the plane and the axis of rotation 30, setat 45 degrees (as illustrated in the particular embodiment of FIGS. 1, 2and 3) the angle 2A between the scan path No. l and No. 2 is 90 degreesand scanning paths No. 1 and No. 2 cross at right angles. It should benoted that the shape of the scanning pattern 35 can be changed by merelychanging the angle A. For example, if a narrower lobe pattern isdesired, the angle A need merely be increased. On the other hand, if thelobe pattern is to be increased, the angle A is decreased. The widerlobe pattern may be desirable for low altitude satellites.

The scan pattern 35 crosses the earths horizon four times (as designatedby points U, V, W and X) for each rotation of the double dove prism 20.In response to the scanning pattern, the amplitude of the output signalsgenerated by the detector 34 switched between two levels, that is, a lowlevel signal while scanning outer space and a much higher level signalwhile scanning the earths atmosphere. The occurance and duration ofsignal pulses generated by the detector 34 as the scan pattern traversesthe earth during scan path No. 1 (between points X and U) and generatedduring scan path No. 2(between points V and W), when compared with thedetection of the reference points 1-4 by the magnetic sensors l6A-16D,provides an indication of the attitude of the vehicle relative to theearths horizon.

FIG. 7 illustrates a three flake infrared detector arrangement includingthe flakes 1, 2 and C. The flake C is mounted within the bolometer withits midpoint positioned on a first or vertical center line 50 of thebolometer 34 and at a midpoint between the flakes l and 2. The flakes 1and 2 are located with their centers located on a second or horizontalcenter line 52 of the bolometer 34. The lines 53 and 54 scribed betweenthe center point of the flake C and the center points of flake 1 and 2define an angle 90 degrees there-between, and an angle of 45 degreesbetween the lines 53 and 54 and the center line 50. The combination ofthe three flake detector arrangement along with the control system ofP16. 9 provides for sun radiation rejection. The detector arrangement issuch, taht at least two flakes are required to be simultaneouslyirradiated at any one time if the signals from the flakes are to beaccepted. The flake C is irradiated simultaneously with either flake 1or with flake 2. This arrangement is illustrated by the portion of thescanning pattern illustrated in FIG. 8. As the scanning patternapproached the crossover point 36 scan along path No. 1, the flake C andflake 1 simultaneously receive radiation from the earths horizon. Whenthe double dove prism is rotated an additional 180 degrees to followscan path No. 2, the image of the earth transmitted to the detector 34is re versed so that the flakes C and 2 simultaneously receive radiationfrom the earths horizon. in the case of radiation from the sun, theoptical arrangement provides that only one flake will be irradiated atany one time and therefore any signals generated by any one of theflakes C, and 1 or 2, is electrically rejected by the control circuit ofFIG. 9 in a manner as hereinafter explained. 1

Electrical signals generated by the flakes 1, 2 and C are converted bythe control system of FIG. 9 into signals corresponding to pitch androll errors. Each of the detector circuits 70, 72 and 74 correspondingto flakes 1, 2 and C respectively, are biased for proper operation bythe bias circuits 80-84, respectively. The output signals from thedetectors 70, 72 and 74 are initially pro cessed by three sets ofidentical circuitry, each including a preamplifier (86-90), a DCrestorer (92-96), a low pass filter (98-102) and a threshold detectorcircuit (104-108).

The signals from the magnetic detectors 16A-16D are applied to separateones of the zero crossover detector circuits 110-116, respectively. Thearrangement is such, that at a rotational angle of the prism 20 of zerodegrees, a signal pulse is applied to the cross-over detector 110, at arotational angle of 90 degrees a signal pulse is applied to thecrossover detector circuit 112, at a rotational angle of l80 degrees asignal pulse is applied to the cross-over detector 114, and at therotational angle of 270 degrees a signal pulse is applied to thecrossover detector circuit 116. The output circuit of the zero crossoverdetector 110 is connected to the set terminal of a flip-flop 118, theoutput circuit of the zero crossover detector 112 is connected to theset terminal of the flip-flop 120, the output circuit of the zerocrossover detector circuit 116 is connected to the reset terminal of theflip-flop 120, and the output circuit of the zero crossover detector 114is connected to the set terminal of the flip-flop 121. The T3 outputcircuit from the flip-flop 120 is connected to the reset input of theflip-flop 118 and also through an inverter circuit 122 to the resetterminal of the flipflop 120. Hence, it can be seen that the flip-flop118 is set at the zero degree rotational angle (reference point 1) andis reset at a 90 degree rotational angle (reference point 2). In asimilar manner the flip-flop 121 is set at the 180 degree rotationalangle (reference poin t 3 a ngi isfreset at the 2-70 dwee rota tionalangle (reference point 3).

The output signals from the threshold detector 104 are applied to oneinput circuit of an AND gate 124, while the other input circuit isconnected to the output circuit T3 of the flip-flop 120. The AND gate124 is only enabled during the portion of the scan between therotational angle of degrees (reference point 2) and 270 degrees(reference point 4). Output signals from the threshold detector circuit106 are applied to one input of an AND gate 126, while the other inputcircuit is connected to the inverter 122. The AND gate 126 is enabledonly during the portion of the scan between the rotation angle of 270degrees (reference point 4) and 90 degrees (reference point 2).

Output signals from the AND gate 124 are applied through an invertercircuit 128 to an input circuit of the AND gates 130 and 132. In asimilar manner output signals from the AND gate 126 are applied throughinverter 134 to an input circuit of the AND gates 136 and 138. A secondinput circuit of each of the AND gates 130-138 is connected to theoutput circuit of the flake C threshold detector 108. Hence the ANDgates 130-138 can not be enabled unless the detector 74 (flake C) isirradiated, thereby rejecting signals from the threshold circuits 104and 106 and providing sun rejection. The third output circuit of the ANDgate 130 is connected to the output T1 of the flip-flop 118, while thethird input circuit to the gate 132 is connected to T1 output circuit.in a similar manner, the third input of the AND gate 136is connected tot he T2 output circuit of the flip-flop 121, while the T2 output isconnected to an input circuit of the AND gate 138.

The output circuit of the AND gate 130 is connected through thedifferential amplifiers 140 and 142 to the reference clamp circuits 144and 146 to produce the signals Hll(+) and H1l(). In a similar manner,the output circuit of the AND gate 132 is connected through thedifferential amplifier circuits 148 and 150 to the reference clampcircuits 152 and 154 to produce the output signals H21(+) and H21(). Theoutput circuit of the AND gate 136 is connected through the differentialamplifiers 156 and 158 to the reference clamp circuits 160 and 162 toproduce signals H12(+) and Hl2(), while the output circuit of the ANDgate 138 is connected through the differential amplifiers 164 and 166 tothe reference clamp circuits 168 and 170 to produce the output H22(+)and H22(). The symbol indicates that a positive signal of a presetamplitude is generated, while the symbol indicates that a negativesignal of preset amplitude is generated. The output signals H11(+),H21(-), H12(+) and H22() are combined to produce a signal S1. The outputsignals H11(+), H21(), H12() and H22(+) are combined to produce anoutput signal S2. The signal S1 is applied through a filter circuit anda lead-lag network 182 to produce the pitch error signals. The signal S2is applied through a filter circuit 184 and the lead-lag network 186 toproduce the roll error signals.

The operation of the control system of FIG. 9 will now be explained withreference to the electrical waveforms of FIGS. 10-12. It is to beunderstood, that the waveforms are not drawn to scale but areexagerated, since the primary purpose of FIGS. 10-12 is to aid in theexplanation of the timing sequence of reference pulses 1-4 relative tothe occurrance of the horizon sensing pulses from the detectors and thecorresponding enabling of the gating circuit to produce the variouspitch and roll error signals. The electrical waveforms are designatedwith reference letters along the left hand side of the Figures, whichprovide a cross reference to corresponding signals from the circuits ofFIG. 9.

As previously mentioned, the timing for the control system is providedby the reference pulses 1-4 induced in the magnetic sensors 16A-l6D asthe double dove prism 20 is rotated. The occurance of the referencepulses (REF1-REF4) are denoted by a sinsusoidal pulse with a zeroamplitude crossover that produces the timing pulse. The waveforms arereferenced to the timing pulses by the verticle lines designated at thetop by REFl-REF4 and at the bottom by the angle of rotation.

FIG. illustrates the electrical waveforms for a proper attitude of thespace vehicle relative to the earth, i.e. zero roll error and zero pitcherror. During scan path No. l flakes 1 and C cross the earths horizonsimultaneously to produce the signals R1 and RC. Shortly thereafter, theflake 2 crosses the earths horizon to produce the signal R2. During thesubsequent scan path No. 2, the flakes 2 and C cross the earth s horizonsimultaneously, while flake l crosses shortly thereafter. Hence thesignals R1, R2 and RC occurring during the period including thereference point REFl (0), are scan path No. l signals while the signalsoccurring during reference point REF3 (180) are scan path NO. 2 signals.The signals R1, R2 and RC are filtered by the filter circuits 98, 100and 102, respectively, to remove any noise and produce the signals F1,F2 and FC. The signals F1, F2 and FC are processed by the thresholddetectors 104-108, respectively, to produce signals El, E2 and EC. Theenabling sequence of AND gates 124, 126, 130, 132, 136 and 138 is underthe control of the output signals from the flip-flops 1 18, I and 121,as designated by the signals T1, T2 and T3, respectively. The AND gate124 is enabled by the signal T3 to pass the signal E1 to produce thesignal Elg. The AND gate 126 is enabled by the signal T3 (the oppositeof signal T3) to pass the signal E2 to produce a signal E2g. The signalsHll(+) and H11() are generated in the response to the simultaneouspresence of signals T1, Elg and EC. The signals H21 and H2l() aregenerated in response to the simultaneous presence of the signals Tl Elgand EC. The signals Hl2() are generated in response to the simultaneouspresence of the signals T2, E2g and EC. The signals H22(+) andH22(-)'are generated in response to the simultaneous presence of thesignals T2 E2g and EC. With zero attitude error, the timing of thesesignals is such that the durations of the signals designated by theletter H are essentially equal, and when they are combined, the signalsaverage out to produce zero pitch error S1, and zero roll error S2.

FIG. 11 illustrates the waveforms of the signals of the control systemof FIG. 9 when a pitch error is present. The relation between thesignals El, E2 and EC from the flakes l, 2 and 3, along with the timingreference pulses REFl-REF4, set the flip-flops 118-121 so that thesignals T1, T1, T2, T2, T3, T3 and EC control the enabling of the ANDgates 124, 126, 130, 132, 136 and 138 in such a manner that the Hsignals are of unequal duration. Note that in the case of pitch error,the signals H'1l(+), Hl1(), H12(+) and H12(-) are substantially shorterin duration than the signals Hl2(+), Hl2(), H22(+) and H22(-). When theH signals are combined, the S2 signal averages out to produce a zeroroll error. However the S1 signal, when averaged out, produces anegative going error indicating the magnitude and the direction of thepitch error.

FIG. 12 illustrates the waveforms for the control system of FIG. 9 inthe case of a roll error and zero pi t ch error As in the case of pitcherror, the signals T1, T1, T2, T2, T3 and T3 and EC'control the timingof the AND gates 124, 126, 130, 132, 136 and 138 sothat differentdurations of H signals are generated. In the case of roll error, thesignals Hll(+), Hl1(), H22(+), H22() are substantially shorter than thesignals H12(+), H12(), H2l(+), and H2l(). When the H signals arecombined, the S1 signal averages to zero indicating a zero pitch error.When the S2 signal is averaged out, a negative going error signal isproduced indicating the magnitude and the direction of the roll error.

Hence, it can be seen that the control system of FIG. 9 is responsive tothe signal pulses generated by the flakes l, 2 and C and the referencepulses REFl-REF4 to produce signals S1 and S2 indicating the magnitudeand direction of any pitch, or roll error, or both. As previouslymentioned, the flake C must be irradicated with either of the flakes lor 2 before the control circuit of FIG. 9 will process the signals toproduce the roll and pitch errors. This is because the AND gates 130,132, 136, and 138 can only be enabled by the presence of the signal ECfrom the threshold detector 108. In the event that radiation is receivedby either of the flakes 1 or 2, or both, without flake C, such as in thecase of radiation from the sun, the signal EC will not be generated andtherefore none of these AND gates will be enabled and no 51 and S2signals will be applied to the lead-lag networks 182 and 186.

FIG. 13 discloses another embodiment of the invention for directing abeam of radiation along the'duallobe crossed-scanning pattern of FIG. 6.A beam of radiation from a laser source is directed by a telescopeincluding lenses 192'and 193 along an axis 194 to the double dove prism20. The axis 194 intersects the axis of rotation 30 and is generallynormal thereto. As the double dove prism 20 is rotated, the beam ofradiation is deflected in a manner so that the beam follows thedual-lobe crossed scanning pattern.

Although the optical scanning system of the invention has been describedin the preferred embodiment including a double dove prism arrangement,it is to be understood that the scanning system will also function witha single dove prism. Such an arrangement canv be exemplified by deletingthe prism 26 from the scanner in FIGS. 1-3, 4A, 4B and 5A and 5B, and bykeeping the prism 24 intact. In such a case, as the shaft 22 is rotated,the single prism 24 will provide the dual-lobe crossed scanning patternillustrated in FIGS. 6 and 8. The ray diagrams of FIGS. 4A, 48, 5A and58 will also apply to the single prism arrangement. The advantage of thedouble dove prism arrangement is that full aperature is provided for thelinear portion of the scanning pattern providing maximum sensitivity. Onthe other hand, the use of the single prism arrangement will .provideessentially the same scanning pattern, but at a reduced sensitivity.

A further modification is the use of a thin double sided reflectingmirror, instead of the prisms 24 and 26. By double sided reflectingmirror we mean a mirror that has a silvered reflective surface on bothsides. Under this arrangement, the shaft will be suitably connected tothe ends of the mirror so that the axis of rotation 30 intersects theplane of the mirror at transverse angle, in a manner as illustrated bythe planar surface 28 of HO. 3. in this case, the outline of the prisms24 and 26 of FIG. 3 can be ignored and the double sides mirror isrepresented by the planar surface 28. Under such an arrangement, as theshaft 20 is rotated, the mirror will provide the same wobbling movement,relative to the axis of rotation 30, as the planar surface 28. Thedetector 34 will again be mounted relative to the axis of rotation 30and the mirror, as it is with the double dove prism reflecting surface28, to receive radiation reflected from the mirror surfaces. For anangle of rotation of the shaft 22 of 180 degrees one side of the mirrorwill face the detector 34, while for the other 180 degrees of radiationthe other side will face the detector. The double side reflecting mirrorwill provide a similar dual-lobe crossed scanning pattern as illustratedin FIG. 6, however having a discontinuity near the zero degree and 180degree etc. points (REFl and REF3) and at a lower effeciency than withthe single or double dove prism arrangement. The discontinuity at highaltitude orbits will present problems, however for low altitude orbits,the discontinuity will be small relative to the duration of a scan passacross the earth (as it is in the center of the earth) and can beignored.

The optical scanning system of the invention has the advantage of havingonly one moving unit thereby reducing lubrication problems and improvingthe statistical life of the system. In addition, the weight of thescanner is reduced as compared to the systems of the prior art therebyreducing problems associated with placing satellites into orbit. Theshape of the lobes in the scanning pattern can be changed by changingthe angle A between the axis of rotation 30 and the reflecting surface28. Only one crossed scan across the earth is required to providesufficient information for controlling the attitude of the vehiclerelative to the earth, and sufficient information to provide altitudeinformation. Since only the portion of the scan pattern near thecrossover is used, a cover with an X shaped transparent pattern can beprovided, thereby minimizing the problems concerned with structuralstrength.

i claim:

1. An optical scanning system comprising:

detection means responsive to radiation for producing an electricalsignal;

optical means for imaging radiation on said detection means, and

scanning means having a dual-lobe crossed scanning pattern fortransmitting the radiation received via the scanning pattern to saidoptical means.

2. An optical scanning system comprising:

detection means responsive to radiation for producing an electricalsignal;

at least one prism; and

means for rotating said prism relative to said detecting means so thatsaid prism produces a dual-lobe crossing type of scanning pattern andtransmits radiation to said detection means received with said scanningpattern.

3. An optical scanner system as defined in claim 2 including:

two prisms mounted to form a double dove prism.

4. An optical scanning system comprising:

a source of radiation, and scanning means for receiving radiation fromsaid source and transmitting the radiation in a dual-lobe crossedscanning pattern. 5. An optical scanning system as defined in claim 4wherein said scanning means includes:

at least one prism, and means for rotating said prism relative to saidsource so that said prism projects the radiation along said dual-lobecrossed scanning pattern 6. An optical scanning system as defined inclaim 4 wherein said scanning means includes:

a double dove prism, and 7 means for rotating said prism relative tosaid source so that said double dove prism projects the radiation alongthe dual-lobe crossed scanning pattern. 7. A horizon sensor forcontrolling the attitude of a space vehicle relative to a space objectcomprising:

detection means responsive to radiation for producing an electricalsignal; scanning means having a dual-lobe crossed scanning pattern forscanning the object and transmitting radiation received from objectscanned via at least a portion of the scanning pattern to said detectionmeans; signal generating means coupled to said scanning means forproviding electrical signals for identify ing the portions of thescanning pattern being traversed, and circuit means responsive to theelectrical signals from the detection means and said signal generationmeans for providing control signals for orienting the attitude of thevehicle relative to the object. 8. A horizon sensor as defined in claim7 wherein said scanning means includes: at least one prism, and meansfor rotating said prism relative to said detection means so that saidprism produces the duallobe crossed pattern. 9. A horizon sensor asdefined in claim 7 wherein said scanning means includes:

a double dove prism, and means for rotating said double dove prismrelative to said detection means so that said double dove prism producesthe dual-lobe crossed scanning pattern. 10. A photoelectric scanningsystem comprising: first optical means including a planar surface havingtwo radiation reflective sides; means for mounting said first opticalmeans for rotation about an axis that is transverse to said planarsurface at an angle other than normal so that said first optical means,when rotated, produces a crossed scanning pattern; radiation sensitivedetection means, and second optical means mounted to alternately receiveradiation from opposite reflective sides of said first optical means assaid first optical means is rotated for directing the radiation soreceived to said detection means. 11. A photoelectric scanning-system asdefined in claim 10 wherein:

said first optical means includes at least one prism. 12. Aphotoelectric scanning system as defined in claim 10 wherein:

said first optical means includes two prisms having a surface of eachabutting to form the planar surface.

13. A photoelectric scanning system as defined in claim wherein:

the optical axis of said second optical means lies at an anglesubstantially normal to the axis of rotation.

14. A photoelectric scanning system as defined in claim 13 wherein:

said planar surface forms an angle between 30 to 60 with the rotationalaxis.

15. A photoelectric scanning system as defined in claim 10 wherein:

said detection means includes three radiation sensitive surfaces forproviding electrical signals in re sponse to radiation applied thereto.

16. An optical scanning system comprising:

optical means providing a planar surface having tow radiation reflectivesides;

means for mounting said optical means for rotation about an axis that istransverse the planar surface at an angle other than normal so that saidoptical means, when rotated about said axis, produces a corssed scanningpattern;

means for rotating said optical means about said axis,

and

means for directing a beam of radiation toward said optical means alonga direction lying in a plane that is transverse said axis of rotation toalternately impinge on opposite reflective sides of said planar surfacewhen said optical means is rotated.

17. An optical scanning system as defined in claim 16 wherein:

said optical means includes at least one prism.

18. An optical scanning system as defined in claim 16 wherein:

said optical means includes two prisms having a surface of each abuttingto form the planar surface.

19. An optical scanning system as defined in claim 16 wherein:

said beam of radiation is directed along a plane that is normal to saidaxis of rotation.

20. A horizon sensing system for space vehicles adapted for controllingthe attitude thereof relative to space objects comprising:

radiation sensitive means for producing electrical signals in responseto radiation applied thereto; optical scanning means having a dual-lobecrossed scanning pattern for directing radiation received from saidscanning pattern as said object is scanned to said radiation sensitivemeans, and

control circuit means receiving said electrical signals from saidradiation sensitive means for generating control signals that are afunction of the attitude of said optical means relative to said object.

21. A horizon sensor for providing signals for controlling the attitudeof a vehicle relative to a space object comprising:

radiation sensitive means for generating an electrical signal inresponse to radiation applied thereto; a double dove prism; mountingmeans for mounting said double dove prism with one surface of each ofthe prisms abutting, and for rotation about an axis extending at anangle transverse said abutting surfaces so that said double dove prismwhen rotated provides a duallobe crossed scanning pattern; optical meanspositioned to receive radiation from the double dove prism and directingthe radiation so received to said radiation sensitive means, and controlmeans connected to receive electrical signals from said radiationsensitive means for providing signals corresponding to the pitch androll attitude of the vehicle relative to the space object. 22. A horizonsensor as defined in claim 21 wherein: said planet is the earth; saidradiation sensitive means includes a three flake infrared bolometer, andthe arrangement being such that the three flake bolometer, said prismand said optical means provides a means for sun radiation rejection. 23.A horizon sensoras defined in claim 21 wherein said control circuitmeans includes:

detection means for detecting the rotational position of the double doveprism, and circuit means synchronized by said detection means forconverting the signals from the radiation sensitive means into signalsproviding an indication of the magnitude and direction of any pitch androll error. 1 t 24. A horizon sensor as defined in claim 23 wherein:said circuit means provides signal pulses having a duration and apolarity that is a function of the pitch and roll errors. 25. An opticalscanning system comprising: radiation detection means; optical meansincluding a surface that is radiation reflecting on opposite sidesthereof; and means for rotating said optical means so that said opticalmeans produces a crossed scanning pattern having a single crossover foreach complete rotation thereof and directs radiation received via saidscanning pattern onto said radiation detection means. 26. An opticalscanning system as defined in claim 25 wherein: 7

said optical means includes at least one prism. 27. An optical scanningsystem as defined in claim 25 wherein:

said optical means include two prisms having a surface of each abuttingto define said planar surface. 28. An optical scanning system as definedin claim 25 wherein:

said optical means includes a double dove prism.

1. An optical scanning system comprising: detection means responsive to radiation for producing an electrical signal; optical means for imaging radiation on said detection means, and scanning means having a dual-lobe crossed scanning pattern for transmitting the radiation received via the scanning pattern to said optical means.
 2. An optical scanning system comprising: detection means responsive to radiation for producing an electrical signal; at least one prism; and means for rotating said prism relative to said detecting means so that said prism produces a dual-lobe crossing type of scanning pattern and transmits radiation to said detection means received with said scanning pattern.
 3. An optical scanner system as defined in claim 2 including: two prisms mounted to form a double dove prism.
 4. An optical scanning system comprising: a source of radiation, and scanning means for receiving radiation from said source and transmitting the radiation in a dual-lobe crossed scanning pattern.
 5. An optical scanning system as defined in claim 4 wherEin said scanning means includes: at least one prism, and means for rotating said prism relative to said source so that said prism projects the radiation along said dual-lobe crossed scanning pattern.
 6. An optical scanning system as defined in claim 4 wherein said scanning means includes: a double dove prism, and means for rotating said prism relative to said source so that said double dove prism projects the radiation along the dual-lobe crossed scanning pattern.
 7. A horizon sensor for controlling the attitude of a space vehicle relative to a space object comprising: detection means responsive to radiation for producing an electrical signal; scanning means having a dual-lobe crossed scanning pattern for scanning the object and transmitting radiation received from object scanned via at least a portion of the scanning pattern to said detection means; signal generating means coupled to said scanning means for providing electrical signals for identifying the portions of the scanning pattern being traversed, and circuit means responsive to the electrical signals from the detection means and said signal generation means for providing control signals for orienting the attitude of the vehicle relative to the object.
 8. A horizon sensor as defined in claim 7 wherein said scanning means includes: at least one prism, and means for rotating said prism relative to said detection means so that said prism produces the dual-lobe crossed pattern.
 9. A horizon sensor as defined in claim 7 wherein said scanning means includes: a double dove prism, and means for rotating said double dove prism relative to said detection means so that said double dove prism produces the dual-lobe crossed scanning pattern.
 10. A photoelectric scanning system comprising: first optical means including a planar surface having two radiation reflective sides; means for mounting said first optical means for rotation about an axis that is transverse to said planar surface at an angle other than normal so that said first optical means, when rotated, produces a crossed scanning pattern; radiation sensitive detection means, and second optical means mounted to alternately receive radiation from opposite reflective sides of said first optical means as said first optical means is rotated for directing the radiation so received to said detection means.
 11. A photoelectric scanning system as defined in claim 10 wherein: said first optical means includes at least one prism.
 12. A photoelectric scanning system as defined in claim 10 wherein: said first optical means includes two prisms having a surface of each abutting to form the planar surface.
 13. A photoelectric scanning system as defined in claim 10 wherein: the optical axis of said second optical means lies at an angle substantially normal to the axis of rotation.
 14. A photoelectric scanning system as defined in claim 13 wherein: said planar surface forms an angle between 30* to 60* with the rotational axis.
 15. A photoelectric scanning system as defined in claim 10 wherein: said detection means includes three radiation sensitive surfaces for providing electrical signals in response to radiation applied thereto.
 16. An optical scanning system comprising: optical means providing a planar surface having two radiation reflective sides; means for mounting said optical means for rotation about an axis that is transverse the planar surface at an angle other than normal so that said optical means, when rotated about said axis, produces a crossed scanning pattern; means for rotating said optical means about said axis, and means for directing a beam of radiation toward said optical means along a direction lying in a plane that is transverse said axis of rotation to alternately impinge on opposite reflective sides of said planar surface when said optical means is rotated.
 17. An optical scanning systEm as defined in claim 16 wherein: said optical means includes at least one prism.
 18. An optical scanning system as defined in claim 16 wherein: said optical means includes two prisms having a surface of each abutting to form the planar surface.
 19. An optical scanning system as defined in claim 16 wherein: said beam of radiation is directed along a plane that is normal to said axis of rotation.
 20. A horizon sensing system for space vehicles adapted for controlling the attitude thereof relative to space objects comprising: radiation sensitive means for producing electrical signals in response to radiation applied thereto; optical scanning means having a dual-lobe crossed scanning pattern for directing radiation received from said scanning pattern as said object is scanned to said radiation sensitive means, and control circuit means receiving said electrical signals from said radiation sensitive means for generating control signals that are a function of the attitude of said optical means relative to said object.
 21. A horizon sensor for providing signals for controlling the attitude of a vehicle relative to a space object comprising: radiation sensitive means for generating an electrical signal in response to radiation applied thereto; a double dove prism; mounting means for mounting said double dove prism with one surface of each of the prisms abutting, and for rotation about an axis extending at an angle transverse said abutting surfaces so that said double dove prism when rotated provides a dual-lobe crossed scanning pattern; optical means positioned to receive radiation from the double dove prism and directing the radiation so received to said radiation sensitive means, and control means connected to receive electrical signals from said radiation sensitive means for providing signals corresponding to the pitch and roll attitude of the vehicle relative to the space object.
 22. A horizon sensor as defined in claim 21 wherein: said planet is the earth; said radiation sensitive means includes a three flake infrared bolometer, and the arrangement being such that the three flake bolometer, said prism and said optical means provides a means for sun radiation rejection.
 23. A horizon sensor as defined in claim 21 wherein said control circuit means includes: detection means for detecting the rotational position of the double dove prism, and circuit means synchronized by said detection means for converting the signals from the radiation sensitive means into signals providing an indication of the magnitude and direction of any pitch and roll errors.
 24. A horizon sensor as defined in claim 23 wherein: said circuit means provides signal pulses having a duration and a polarity that is a function of the pitch and roll errors.
 25. An optical scanning system comprising: radiation detection means; optical means including a surface that is radiation reflecting on opposite sides thereof; and means for rotating said optical means so that said optical means produces a crossed scanning pattern having a single crossover for each complete rotation thereof and directs radiation received via said scanning pattern onto said radiation detection means.
 26. An optical scanning system as defined in claim 25 wherein: said optical means includes at least one prism.
 27. An optical scanning system as defined in claim 25 wherein: said optical means include two prisms having a surface of each abutting to define said planar surface.
 28. An optical scanning system as defined in claim 25 wherein: said optical means includes a double dove prism. 